1. Field of the Invention
The present invention relates to a color temperature conversion apparatus and method thereof, and more particularly, to a color temperature conversion apparatus and method thereof capable of variably calculating an output color temperature from an input color temperature of an input image. The present application is based on Korean Patent Application No. 2001-51268, filed Aug. 24, 2001.
2. Description of the Related Art
When a person views an object, the person generally perceives that color tone varies based on illumination characteristics. In other words, while one may perceive that the color tone of an object under an incandescent lamp is generally reddish, the color tone is comparatively bluish in daylight. Therefore, in a color display system, there is a need to change the color of an object input into the system, to a color that a person sees. The color tone that is generally shown relates to a color temperature. The term color temperature refers to the temperature at which a black body will emit a light of the same
When a person views an object, the person generally perceives that color tone varies based on illumination characteristics. In other words, while one may perceive that the color tone of an object under an incandescent lamp is generally reddish, the color tone is comparatively bluish in daylight. Therefore, in a color display system, there is a need to change the color of an object input into the system, to a color that a person sees. The color tone that is generally shown relates to a color temperature. The term color temperature refers to the temperature at which a black body will emit a light of the same color as that of a given source, and is measured in Kelvin [K] units. If the color temperature is high, then the color tone is bluish. If the color temperature is low, then the color tone is reddish.
A color display system is broadly used for apparatuses that need to transmit visual information to a user. The apparatuses include, for example, a TV, a DTV (digital TV), a PDP (Plasma Display Panel), a TFT (Thin Film Transistor) monitor, a color printer, a digital camera, a projector, a mobile phone, and so on. The color temperature should be adjusted to improve the quality of a picture in the color display system.
Color models include a RGB (red, green, blue) model, a Hue, Saturation, Brightness/Hue, Lightness, Saturation (HSB/HLS) model, a Munsell color system, and a Commission Internationale de l'Eclairage, or CIE, color model. The CIE color model is set by The International Commission on Illumination, which defines lighting standards. The CIE color model includes color models CIEXYZ, CIELUV, and CIELAB. The CIEXYZ color model expresses RGB values as a set, X, Y, Z, of different values, all having positive signs. Generally, the X, Y, Z values are referred to as tri-stimulus.
A conventional method of converting the color temperature uses the CIEXYZ color model, and more particularly, it is a color temperature conversion method using both the X-axis and Y-axis.
FIG. 1 is a flowchart showing the conventional color temperature conversion method.
A detailed procedure of the conventional color temperature conversion method has been described in “Color Science: concepts and methods, quantitative data and formulas”, 2nd edition, pp. 145–146 & pp. 225–228, 1982, by Gunter Wyszecki and W. S. Stiles. According to the conventional color temperature conversion method, when an image is input (S10), the color temperature Tc of the input image is calculated (S20). Then, a daylight spectrum related to the color temperature Tc of the calculated input image is calculated (S30). The daylight spectrum is calculated by following equations 1) and 2) in accordance with a range of the color temperature Tc of the input image.
1) When the color temperature of the input image is within a range of 4000 K≦Tc<7000 K, the daylight spectrum XD is calculated by,
                              X          D                =                                            -              4.067                        ⁢                                                  ⁢                                          10                9                                            T                C                3                                              +                      2.9678            ⁢                                                  ⁢                                          10                6                                            T                C                2                                              +                      0.09911            ⁢                                                  ⁢                                          10                3                                            T                C                                              +          0.244063                                    [Equation 1]            
2) When the color temperature of the input image is within a range of 7000 K≦Tc<25000 K the daylight spectrum XD, YD is calculated by,
                                                                        X                D                            =                            ⁢                                                                    -                    2.0064                                    ⁢                                                                          ⁢                                                            10                      9                                                              T                      C                      3                                                                      +                                  1.9018                  ⁢                                                                          ⁢                                                            10                      6                                                              T                      C                      2                                                                      +                                  0.24748                  ⁢                                                                          ⁢                                                            10                      3                                                              T                      C                                                                      +                                                                                                      ⁢              0.237040                                                                                          Y                D                            =                            ⁢                                                                    -                    3                                    ⁢                                                                          ⁢                                      X                    D                    2                                                  +                                  2.87                  ⁢                                                                          ⁢                                      X                    D                                                  -                0.275                                                                        [Equation  2]            
A spectrum radiant power distribution SD (λ) of the daylight is calculated by,SD(λ)=S0(λ)+M1S1(λ)+M2S2(λ)  [Equation3]
where, [S0(λ),S1(λ),S2(λ)] correspond to [sO[31], s1[31], s2[31]], and sO[31], s1[31], s2[31] are fixed values, while
      M1    =                            -          1.3515                -                  1.7703          ⁢                      x            D                          +                  5.9114          ⁢                      y            D                                      0.0241        +                  0.2562          ⁢                      x            D                          -                  0.7341          ⁢                      y            D                                    M2    =                  0.0300        -                  31.4424          ⁢                      x            D                          +                  30.0717          ⁢                      y            D                                      0.0241        +                  0.2562          ⁢                      x            D                          -                  0.7341          ⁢                      y            D                              
X, Y, Z of a Macbeth color checker (MCC) is calculated (S40). The MCC obtains the X, Y, Z of each path from the following mathematical equation 4. At this time, the applied MCC has thirty-one (31) elements when a wavelength (λ) has a range of 400 nm to 700 nm. The interval of each element is 10 nm.
                                          [                                                                                X                    i                    ME                                                                                                                    Y                    i                    ME                                                                                                                    Z                    i                    ME                                                                        ]                                3            ⁢                                                  ⁢            X1                          =                                                                                                  [                                                                                                                                                      x                              _                                                        ⁡                                                          [                              i                              ]                                                                                                                                                                                                                                      y                              _                                                        ⁡                                                          [                              i                              ]                                                                                                                                                                                                                                      z                              _                                                        ⁡                                                          [                              i                              ]                                                                                                                                            ]                                                        3                    ⁢                    X31                                                  ⁡                                  [                                      R                    ⁡                                          [                      i                      ]                                                        ]                                                            31                ⁢                X31                                      ⁡                          [                              E                ⁡                                  [                  i                  ]                                            ]                                            31            ⁢            X1                                              [Equation  4]            
where, i=1, 2, . . . , 31, and
      [                                                      x              _                        ⁡                          [              i              ]                                                                                      y              _                        ⁡                          [              i              ]                                                                                      z              _                        ⁡                          [              i              ]                                            ]        3    ⁢    X31  is the function value of color matching, and
[R[i]]31×31 is a reflectance diagonal matrix related to one color path, and each wavelength band value exists only in the diagonal term of the matrix. Off-diagonal terms are all zero, and
[E[i]]31×1 is a spectrum power dispersion of the illumination.
XiME, YiME, and ZiME are MCC tri-stimulus values related to the color temperature Tc of the input image.
The X, Y, Z of the MCC of target color temperature is selected (S50).
A converting matrix is calculated (S60) according to the following mathematical equation 5, using correlation of the color temperature Tc of the input image and the target color temperature.[TM]3×3=[(XYZ)MT]−1 3×24[(XYZ)ME]3×24  [Equation 5]
where, [TM]3×3 is a converting matrix, and
[(XYZ)MT]−13×24 is a matrix of MCC tri-stimulus values X, Y, Z related to the target color temperature, and
[(XYZ)ME]3×24 is a matrix of MCC tri-stimulus values X, Y, Z related to the color temperature Tc of the input image.
The input image is converted to an image of the target color temperature (S70). In other words, it is converted to an image of a target illumination by using the calculated converting matrix.
                              [                                                                      X                  t                                                                                                      Y                  t                                                                                                      Z                  t                                                              ]                =                              [            TM            ]                    ⁡                      [                                                            X                                                                              Y                                                                              Z                                                      ]                                              [Equation  6]            
where, Xt, Yt and Zt are the tri-stimulus elements according to the target color temperature, and
[TM] is the converting matrix, and
X, Y and Z are the tri-stimulus elements according to the color temperature Tc of the input image.
As described above, the image that is converted to the target color temperature is output (S80) to various color display apparatuses.
In the conventional color temperature conversion method, as described above, the tri-stimulus values X, Y, Z of the MCC related to the color temperature Tc of the input image and the tri-stimulus values X, Y, Z related to the target color temperature, should be calculated to draw out the color temperature converting matrix. The number of matrices needed to realize the conventional color temperature conversion method in hardware is shown in table one.
TABLE 1Type of MatrixNumber Needed31 × 1 324 × 311 3 × 31124 × 3 1
Also, a matrix inversion part is needed to obtain the converting matrix. In addition, a plurality of multipliers and dividers are needed for calculating the above equations. Therefore, applying the conventional color temperature Tc conversion method in hardware generates a problem of low practicality and utility in terms of size and costs.
Meanwhile, when displaying one screen, since the conventional color temperature converting apparatus converts one frame of the input image into a predetermined fixed target color temperature, a fine adjustment of color temperature to one frame cannot be guaranteed.